Bimetallic carbon nanocatalysts for methanol steam reforming in conventional and membrane reactors

Article abstract of DOI:10.1016/j.cattod.2016.01.003

In this paper we report the results obtained by a comparative study of catalytic activity of bimetallic (Ru-Rh, Ni-Cu) catalysts synthesized on a surface of different carbon supports: detonation nanodiamonds (DND), infrared pyrolyzed polyacrylonitrile, ca

Full text of DOI:10.1016/j.cattod.2016.01.003

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Contents lists available at ScienceDirect
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Bimetallic carbon nanocatalysts for methanol steam reforming in
conventional and membrane reactors
a,∗
a
a
Aleksandra A. Lytkina , Natalia V. Orekhova , Margarita M. Ermilova ,
b
b
a
a
Sergey V. Belenov , Vladimir E. Guterman , Mikhail N. Efimov , Andrey B. Yaroslavtsev
A.V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Leninsky Pr. 29, Moscow, Russia
a
b
Chemistry Department, Southern Federal University, Zorge St., 7, Rostov-on-Don, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper we report the results obtained by a comparative study of catalytic activity of bimetallic
Received 10 September 2015
Received in revised form 1 December 2015
Accepted 14 January 2016
Available online xxx
(
(
(
Ru–Rh, Ni–Cu) catalysts synthesized on a surface of different carbon supports: detonation nanodiamonds
DND), infrared pyrolyzed polyacrylonitrile, carbon black Vulcan-XC-72, in steam reforming of methanol
MSR) for hydrogen production. All obtained catalysts were characterized by N2 physisorption, SEM, TEM,
FTIR spectroscopy, X-ray diffraction. It was shown, that activity of catalysts and products distribution
depends on metal type, support material and the specific surface area. Support effect was investigated
for Ru–Rh system. DND showed the best properties as a catalyst support. Ru–Rh system is more active
than Cu–Ni as it is shown on the example of DND supported catalysts. Comparative study between
conventional (CR) and membrane (MR) reactors was carried out. MR represented a flow system with
a plane Pd-containing membrane. The effect of different catalysts on the MR performances as well as a
general comparison of the experimental results was considered. Obtained results demonstrate the ability
of the membrane to increase the reaction conversion. The hydrogen stream produced from the MR is ultra
pure: especially, it is CO-free and thus suitable to be directly fed to a polymer electrolyte membrane fuel
cell.
Keywords:
Hydrogen production
Methanol steam reforming
Carbon
Bimetallic catalyst
Membrane catalytic system
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
gen is unsuitable for low-temperature fuel cells due to poisoning
of its catalyst even in the presence of trace impurities of CO [7,8].
Steam reforming of alcohols, which are much more chemically
active than methane, proceeds at considerable lower temperatures
[8–12]. It is important that alcohol, produced from biomass (bio-
alcohol), can be considered as a renewable raw material. Methanol
has a number of advantages compared with other precursors for
hydrogen production. Since it has only one carbon atom, methanol
is one of the simplest compounds that dramatically reduces the
number of reaction byproducts. The absence of the C C bonds in
the molecule allows to conduct reforming at relatively low temper-
Atmosphere of big cities becomes more polluted by products
of incomplete combustion, nitrogen and sulfur oxides as a result
of technical progress. This significantly affects on ecology. From
this viewpoint, an attractive technology is a conversion of chemical
energy into electrical one by fuel cells (FC) which produces water
only. However, absence of hydrogen in nature in a pure form defines
urgency of the problem of its synthesis [1]. Methane can be consid-
ered as a promising feedstock for technical hydrogen obtaining.
Nevertheless, its nonoxidative coupling is accompanied by signif-
icant energy consumption and occurs at high temperatures with
low productivity or requires a forced activation [2]. Steam reform-
ing of methane [3,4], or its high-temperature conversion, flowing
due to partial oxidation by oxygen, are much more economically
feasible [5]. However, process leads to a syngas evolution, wherein
the ratio of hydrogen to CO is usually less than 2:1 [6]. Such hydro-
◦
atures (200–350 C) [13]. Methanol is a toxic substance, but at the
same time it is biodegradable, it is a liquid at ambient conditions.
In the MSR in addition to the target Reaction (1):
◦
= 49.7 kJmol−1
CH OH + H O ↔ CO +3H , ꢀH
(1)
3
2
2
2
298K
also the reactions of methanol decomposition (2), and water-gas
shift (WGS) Reaction (3) occur:
◦
K = 90.2 kJmol−1
CH OH ↔ CO + 2H , ꢀH
(2)
(3)
3
2
298
∗ Corresponding author.
E-mail address: lytkina@ips.ac.ru (A.A. Lytkina).
◦
= −41.2 kJmol−1
CO + H O ↔ CO +H , ꢀH
2
2
2
298K
http://dx.doi.org/10.1016/j.cattod.2016.01.003
0
920-5861/© 2016 Elsevier B.V. All rights reserved.
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Fig. 1. Scheme of the conventional flow reactor.
Fig. 2. Scheme of the membrane reactor.
Reactions (1) and (2) require heat supplying, and Reaction (3)
occurs with heat evolution.
content of noble metals [25], as well as materials containing several
metals [26–28].
As a result a mixture of hydrogen and carbon oxides is obtained;
the ratio of products depends on the process conditions and the
catalyst used.
Carbon monoxide is usually an undesirable product. Its forma-
tion according to Reaction (2) reduces hydrogen yield. The problem
of reducing the amount of carbon monoxide is very crucial because
it poisons the Pt-containing catalysts in a fuel cell. It imposes strict
requirements to hydrogen purity and emphasizes the importance
of the catalyst choice for MSR reaction [5,7].
An important issue is to increase selectivity of the composite and
its resistance to carbonization. Therefore, new fine carbon materi-
als are of great interest, such as nanotubes, nanowires, graphene
and nanodiamonds (DND). The unique properties of these materi-
als make them appropriate for use as catalyst supports [12,29,30]. It
is important that application of carbon carriers mainly eliminates
the problem of a catalyst carbonization. Moreover DND or fine-
dispersed carbon black have high specific surface area and mainly
mesoporous structure.
Deactivation of catalysts for MSR can occur generally owing to a
change in oxidation state, sintering of metal particles or coke depo-
sition. It is known that resistance to deactivation of monometallic
catalysts can be improved by adding promoters [14]. Cu-containing
composites with various additives that increase the stability of the
catalyst and the hydrogen yield are considered as the good catalysts
for the MSR. For example, addition of Ni to the copper monometallic
catalyst significantly increases its resistance to deactivation [15]. It
was also shown that noble metals such as Rh, Ir, Pt, Ru, Pd, have
high activity in steam reforming of alcohols [16]. Moreover, Rh is
regarded as one of the most active in a number of noble metals
The problem of high purity hydrogen production (which is
needed for PEMFC) can be successfully solved in the case of using of
membrane catalysis by selective removal of hydrogen [13,31,32].
During the last few decades a large number of researches were
devoted to carrying out of alcohols SR in membrane reactors (MR)
[33–37]. The advantage of this approach lies not only in production
of impurities free hydrogen, but in an increase of its yield also by
selective extraction from reaction zone.
Compared with pure Pd membranes, membranes, made of some
Pd alloys, have several advantages because of their higher hydro-
gen permeability, strength, thermal stability and catalytic activity
[38–42]. The introduction of even small amounts (∼1 wt%) Ru into
palladium increases mechanical stability of the membrane during
thermal cycling in hydrogen [43].
[
17]. In the works [18–22] Ru is considered as a promising cata-
lyst for alcohol steam reforming processes also. The authors of [23]
have reported a significant selectivity improvement of catalysts
doped by Rh, owing to its activity in the WGS reaction. However,
application of noble metals in catalysis is limited because of their
high-value [24]. Therefore, of great interest are catalysts with a low
The aim of the work was to search new productive and selec-
tive to hydrogen catalysts for MSR on various carbon supports
(DND, IR pyrolyzed polyacrylonitrile, carbon black Vulcan) and to
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◦
Fig. 3. XRD patterns of the catalysts in the region of 2ꢁ = 10–90 : (a) Ru–Rh/DND; (b) Ru–Rh/PAN; (c) Ru–Rh/Vulcan.
◦
study of obtained catalysts behavior in the conventional tube and
membrane reactors. Comparing of catalytic activity in MSR reac-
tion of Ru–Rh and more cheap Ni-Cu systems, supported on DND
was of interest also.
annealed at 400 C in inert atmosphere for 3 min. Further, they were
◦
treated with H2 (5%)/Ar (20 ml/min) for 3 h at 350 C. Thus, three
bimetallic Ru–Rh/C samples were prepared, differing in nature of
the support and the catalyst Ni–Cu/DND.
Steam reforming of methanol was carried out at atmospheric
pressure in the conventional stainless steel tubular reactor (Fig. 1)
◦
2
. Experimental
in a temperature range of 200–400 C. The tube length was of
1.5 cm and inner diameter was of 0.9 cm. A catalyst sample (load-
2
2
.1. Synthesis of the catalysts and technique of the catalytic
ing was 0.3 g) was mixed with granular silica and placed to the
middle of the reactor tube. Heating of the reactor up to a work
temperature was carried out by electric furnace during constant
experiments
3
Ru–Rh/C catalysts were prepared by reduction of metals precur-
argon flow (20 cm /min); the temperature of the reactor was mea-
sors (ruthenium(III) chloride and rhodium (III) chloride) at room
temperature in a liquid phase using 0.5 M NaBH4 as a reducing
agent. As a medium for the synthesis aqueous solution of ethy-
lene glycol (EG 50% by volume) was used. As support detonation
nanodiamonds, Vulcan XC-72 and IR pyrolyzed polyacrylonitrile
were used. Calculated amount of carbon support was added into
the metal-containing precursor solution and dispersed using an
ultrasonic homogenizer US Soniprep to obtain a homogeneous sus-
pension. The amount of precursors was calculated based on the
total metal content in a material after their complete reduction of
sured by a chromel–alumel thermocouple. Catalysts were reduced
3
in situ with a mixture of H2 (5%)/Ar (20 cm /min) at a tempera-
◦
ture of 350 C for 3 h before every experiment. During the reaction,
3
argon flow rate was constant and equaled to 20 cm /min. A liq-
uid mixture of methanol and water, with a stoichiometric molar
ratio of 1:1 was fed to an evaporator using an infusion pump Insti-
lar 1488 Dixion. Unreacted alcohol and water were condensed in
◦
a glass collector, cooled to +1 C. Analysis of uncondensed reaction
products was performed by gas chromatograph LHM 8MD with a
thermal conductivity detector, helium as a carrier gas and columns
with stationary phases Porapak T for water, methanol and other
7
.5 wt.% and metals atomic ratio of 1:1. The mixture was brought up
to pH 10 by an aqueous solution of ammonia, and then during inten-
sive stirring threefold excess of a freshly prepared aqueous solution
of 0.5 M NaBH4 was added. The solution was kept for 40 min under
constant stirring for complete reduction of both metals. Obtained
suspension was filtered using a Buchner funnel, washed repeat-
oxygenates and activated carbon for CO and CO . Concentrations
2
of hydrogen and methane in products were measured by a chro-
matograph Chrom-4 with a thermal conductivity detector and a
CaA Zeosorb column, with Ar as a carrier gas. Chromatograms were
processed using software package Ecochrom (Fig. 1).
edly by deionized water and acetone. A resulting powder was dried
The second series of MSR experiments was carried out in the
MR. The samples of catalysts mixed with granulated quartz (frac-
tion 1–3 mm) were placed into the reaction zone of the MR, which
scheme is shown on a Fig. 2. The reactor consisted of two stainless
steel cylindrical-shaped compartments, divided by a pinhole-free
foil membrane of Pd–Ru alloy (6 mass% Ru). The thicknesses of
investigated membranes were 12 and 70 microns. Two gaskets
made of copper and of Graflex, ensured a hermeticity of the reac-
tor. Experiments in MR were carried out at the temperatures of
◦
at 80 C for 3 h. Obtained catalysts were treated with H (5%)/Ar
2
◦
(
20 ml/min) for 3 h at 350 C for metal oxides reduction.
For
a synthesis of the Ni–Cu/DND catalyst DND pow-
der was sequentially impregnated with aqueous solutions of
Cu(NO ) ·3H O (>98%) and Ni(NO ) ·6H O (≥98.5% ALDRICH) at an
3
2
2
3
2
2
appropriate concentration calculated so that the total metal con-
tent was equal to 20% from the support weight; nickel and copper
ratio was 1:4 (previously it has been shown that this ratio is close
to optimal for the system Ni–Cu [44]). So-prepared suspension was
dispersed in an ultrasonic bath for 1 h, and then excess of water was
◦
200–350 C. The flow rate of sweep gas on a permeate and reten-
◦
evaporated at 100 C in a drying oven. The obtained catalysts were
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3
tate side was of 20 cm /min. The reaction products were analyzed
by the method described above.
The alcohol conversion degree, X (%) was calculated from the
results of analysis by using the following equation:
X = [
ϕ − ϕ ]
× 100
0
1
ϕ
0
where ϕ0 and ϕ1 are the initial and the final ethanol concentration
respectively.
The yields of products was assessed as the amount of the corre-
sponding reaction product (in moles) forming per gram of metal in
the catalyst per hour.
2.2. Structure and morphology of the catalysts
Fig. 4. XRD patterns of the catalyst: Ni–Cu/DND.
The specific surface area and pore size were determined using
the N adsorption BET method with an ASAP-2020N (Micromeritics
2
Co., USA) instrument. X-ray diffraction analysis (XRD) of the sam-
ples was performed on X-ray diffractometer Rigaku D/Max-2200,
CuK␣1—radiation. For spectra processing and qualitative analysis
Rigaku Application Data Processing software package was used.
Particles sizes (coherent scattering region) were estimated from
the width of the XRD patterns peaks by Scherrer equation:
k × ꢂ
d =
(4)
(
B − b) × cos ꢁ
where k = 0.89—Scherrer constant; ꢂ = 1.5406 Å—the wave length of
used radiation; B—a width at half maximum(2ꢁ); b—instrumental
broadening (2ꢁ); ꢁ—the angle of a peak position.
TEM images were taken with a Transmission Electron Micro-
scope JEM 2100 with an acceleration voltage of 200 kV, with a point
resolution 0.23 nm. The chemical analysis data and SEM images
were obtained using the Scanning Electron Microscopy technique
on Carl Zeiss NVision 40 with adapter for element analysis; an
acceleration voltage was 200 kV.
Fig. 5. SEM image of the catalyst: Ru–Rh/DND.
◦
with maxima at 43.6 corresponding to nanodiamonds (Fig. 3a) and
IR spectra of the supports in the form of pellets with Ge were
recorded before using in MSR process by using the FTIR spectrom-
amorphous halo of IR PAN (Fig. 3b) are seen. It can be noted that
carbon black Vulcan has some order in its structure, manifested in
broad reflections with peaks at 25, 43 and 78.5 (Fig. 3c). Narrower
−
1
◦
eter IFS-66-v/s in the wavelength range 600–4000 cm
.
peaks correspond to alloy Ru–Rh, reflexes maximums of this alloy
are located between the positions which are characteristic for the
individual metals. The unit cell parameters for the resulting alloys
are equal to 0.38044 ± 0.0002 nm (Fig. 4).
3
. Results and discussion
3.1. Structural and morphological characterization of the
catalysts
The XRD pattern of the catalyst Ni–Cu/DND are similar to
described above (Fig. 4). It is possible also to identify the broad lines
of nanodiamond and peaks of Ni–Cu alloy with unit cell parameter
of 0.3616 ± 0.0009 nm.
Under the study we have investigated three types of carbon sup-
ports differing in surface area and nature of functional groups on
the surface:
Table 1 shows the specific surface area of the supports, the
composites, micropore area, and the average particle sizes of com-
posites estimated by the BET method and average metal particle
sizes, estimated from the width of the XRD (coherent scattering
region (CSR)). Obviously, the specific surface area is determined
by dispersion degree of a carbon support. Thus, the specific sur-
face area decreases in a line of PAN » DND> Vulcan. Average particle
sizes, estimated by BET technique, correspond mainly to average
sizes of carbon particles. This follows from its higher content. At the
same time, sizes of metal particles are similar in all cases, according
to the XRD reflexes widths. It should be noted also that significantly
larger specific surface area of PAN, according to BET data, primarily
determined by its porosity.
According to the SEM data, all samples represent tracery
agglomerates with a size of 50–300 nm, consisting of substantially
smaller particles (Fig. 5). From the data obtained by microprobe
analysis, the ratio of ruthenium and rhodium in composites corre-
sponds to 1.5:1, which is slightly higher than the initial load. The
ratio of nickel and copper in the sample Ni–Cu/DND corresponds
to initial load of 1:4.
1
) Detonation nanodiamond, which is a product of detonation syn-
thesis. DND nanoparticles consist of a core, having diamond
lattice (carbon atoms in sp3-hybridisation), and amorphous car-
bon shell with a thickness up to four carbon atoms. Furthermore,
DND surface contains a large amount of oxygen-containing func-
tional groups that provide good adsorption properties of the
material [45].
2
) IR pyrolyzed polyacrylonitrile, this material was prepared by IR
annealing of polyacrylonitrile (PAN) at the temperature of about
◦
8
00 C, and its subsequent activation by potassium hydroxide.
IR PAN is a microporous material with a high surface area and it
contains carbon with different hybridization and structure.
) Carbon Black Vulcan is a commercial product obtained by
pyrolysis of gaseous hydrocarbons. Vulcan contains globular
amorphous carbon with a basic particle size of about 30 nm.
3
Fig. 3 shows X-ray patterns of catalysts supported on differ-
ent types of carbon materials. On roentgenograms broad peaks
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Fig. 6. TEM images of the Ru–Rh/DND (a, b) and Ni–Cu/DND (c) catalysts.
Table 1
The specific surface area and average nanoparticle sizes of obtained materials.
Support
Catalyst
Average particle size (nm)
2
2
2
BET surface area (m /g)
286 ± 3
BET surfacearea (m /g)
Micropore area (m /g)
BET
20
25
30
4
CSR (metals)
13 ± 1
DND
Ru–Rh/DND
Ni–Cu/DND
Ru–Rh/Vulcan
Ru–Rh/PAN
289 ± 3
230 ± 3
206 ± 3
1600 ± 3
15 ± 3
14 ± 3
12 ± 1
Vulcan
PAN
200 ± 3
67 ± 3
13 ± 1
1880 ± 3
1124 ± 3
13 ± 1
◦
TEM data provide more information about the obtained sam-
ples. They evidence that the average particles size of metals is
of 5–15 nm in all studied composites (Fig. 6). Note that this size
is somewhat smaller than value, estimated from the X-ray peaks
width, which probably is a result of a higher reflections intensity of
roentgenograms from the larger particles. Similar patterns are dis-
cussed in detail in [46]. At the same time the atomic layers of Ru–Rh
alloy with interplanar distances, corresponding to XRD reflexes, are
good seen on separate micrographs.
catalyst Ru–Rh/PAN only at 350 C, but in this case, the amount of
emitted CO is 30% higher than in the case of DND.
Obtained results indicate a significant influence of a support
nature on the rate and selectivity of steam reforming of methanol.
The lowest activity of the catalysts based on carbon black Vul-
can could be explained by less developed surface area. However
the highest activity of the catalysts supported on nanodiamonds,
compared with PAN contradicts this assertion. In this case, the
specific surface area of nanodiamonds approximately 6.5 times
lower. It could be assumed that nanodiamonds have their own cat-
alytic activity in MSR reaction. Indeed, we have previously observed
a similar property of DND in the process of steam reforming of
ethanol [12]. Own catalytic activity is observed for them, in case of
MSR too (see. Fig. 8a). However, its magnitude is more than two
orders lower than one obtained on Ru–Rh containing catalysts at
equivalent temperatures.
The obtained data leads to the conclusion that both a metal and
the carrier involved in the catalytic process. FTIR spectra of PAN
and Vulcan (Fig. 9, curves 2 and 3) are nearly identical and cor-
respond to highly oxidized carbon with a high content of C
bonds, most likely in cyclic unsaturated compounds with a low con-
tent of conjugated bonds on the surface (1580, 1485 cm ). There
is a small amount of saturated CH bonds (2800–2980 cm ) as well
as carbonyl groups (1730–1710 cm ), but their content is negli-
gible. The hydroxyl groups on the surface of Vulcan and PAN are
practically absent —there are no bands in the region of 3400 and
3.2. Activity of the catalysts in steam reforming of methanol
3.2.1. Conventional reactor
According to the obtained data, the achieved methanol con-
version was not lower than 85% for all catalysts at temperatures
◦
of about 300 C. With further temperature increase the process
becomes less selective toward to the hydrogen production. Fig. 7
shows the temperature dependences of hydrogen and carbon
monoxide flows on the Ru–Rh/DND and Cu–Ni/DND catalysts.
It is easy to see that the catalyst based on Ru–Rh alloy differs
with significantly higher activity as compared with Cu–Ni. At the
same time, it should be noted that at low temperatures Cu–Ni/DND
catalyst is more selective. Equivalent flow of CO on Cu–Ni catalyst
O C
−
1
−
1
−
1
◦
is achieved at temperatures of about 60 higher than on Ru–Rh. At
◦
temperatures lower than 270 C mostly the Reaction (1) proceeds
−
1
on Cu–Ni catalyst, and produced hydrogen is substantially free of
CO.
1630 cm
.
The surface of nanodiamonds contains a large amount of
hydroxyl groups. Position of C O bonds in the spectrum is shifted
At the same time, nature of the support plays an important role
in MSR mechanism. Fig. 8 shows plots of hydrogen flow rate of the
temperature on supported catalysts Ru–Rh/DND, Ru–Rh/Vulcan
and Ru–Rh/PAN. Among the composites the highest catalytic activ-
ity was shown by the catalyst Ru–Rh/DND. Herewith activity of the
catalysts decreased in the line of DND> PAN> Vulcan. At the same
time, the catalyst supported on carbon black Vulcan was the most
selective toward the process, proceeding according to the Reaction
−
1
to shorter wavelengths (950 cm ). This evidences that C O bonds
on the DND surface are part of the functional groups COOH, COOR.
Ether bonds are present also because the spectrum of DND exhibits
disintegration of the band. These surface species can actively adsorb
water molecules by formation of hydrogen bonds like H
O
H· · ·
O. In turn, methanol molecules are adsorbed on the metal surface
(Fig. 10). Then an activated oxygen of water heads to a carbon atom
of methanol and results in carbon dioxide molecules formation.
Obviously, this process can only takes place at the metal-carrier
boundary, the development of which is determined only by the
(
1).
Maximum amount of hydrogen at 300–310 C was obtained on
◦
the DND supported catalyst, which is 2.3 moles from 3 theoretically
possible. The similar amount of hydrogen can be obtained on the
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Fig. 7. Temperature dependence of (a) H2; (b) CO yields in MSR on the Ru–Rh/DND and Ni–Cu/DND catalysts.
Fig. 8. Temperature dependence of (a) H2; (b) CO yields in MSR on the obtained catalysts.
Fig. 10. Scheme of methanol steam reforming process over catalysts surface.
tions (2) and (3) in the opposite direction increases due to the
thermodynamic factors.
The reason for the higher activity of Ru–Rh catalyst in MSR com-
pared to Ni–Cu, probably is a stronger adsorption of methanol.
This leads to easier destruction of methanol molecule. On the other
hand, it is also leads to an even higher rate of methanol decompo-
sition by Reaction (2) with the release of CO. Nevertheless authors
[
47] explained it by the fact that on copper surface single-center
methanol adsorption via an oxygen atom is preferably, while on the
surface of metals of 8–10 groups two-center adsorption also takes
place via the of oxygen and carbon atoms. However, this approach,
in our opinion, does not explain the significant dependence of cat-
alytic activity on a support nature.
Fig. 9. IR-spectra of DND (1), PAN (2) and Vulcan (3).
amount of a supported metal and its particle size, and in the first
approximation should not depend on support surface area. Another
important factor is a concentration of oxygen-containing groups
on the surface of the carbon support. All this together determines
a higher activity of Ru–Rh catalyst supported on nanodiamonds.
During the temperature rise the probability of proceed of Reac-
3.2.2. Membrane reactor
One of the most promising approaches is to carry out cat-
alytic processes directly in a MR [35,36]. In this case, the use
of palladium-containing membranes allows to obtain high-pure
hydrogen directly in a catalytic reactor. The examples of such mem-
brane catalytic systems for ESR were recently reported for metal
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Fig. 11. The flow rates of H2 in a retentate (curve 1) and permeate zones (curve 2), the total hydrogen flow (curve 3) in the membrane reactor, and in the conventional reactor
(
curve 4) on the catalysts (a) Ru–Rh/DND; (b) Ru–Rh/PAN; (c) Rh–Ru/Vulcan.
Fig. 12. The flow rates of CO in the membrane and conventional reactors on the catalysts (a) Ru–Rh/DND; (b) Ru–Rh/PAN; (c) Rh–Ru/Vulcan.
catalysts on different supports [36] and for catalytic nanocompos-
ites [33].
roots of hydrogen partial pressures in the retentate and permeate
zones. Such dependence is called Sieverts law. In the case of thin
membranes hydrogen permeation through the membrane is accel-
erated and its speed is mostly limited by adsorption processes [48].
At the same time, force of Siverts law stopped and the effective
value of hydrogen permeability through the membrane decreases.
The reasons for deviations from Sieverts law may be poisoning the
surface with CO, and the surface processes such as “concentration
polarization” [49,50]. This phenomenon is concluded in difficulty of
Fig. 11 shows the dependence of hydrogen flow rate, obtained
in the MR, on temperature with use of the DND, Vulcan and PAN
supported catalysts. The total hydrogen yield on all investigated
catalysts at high temperatures was approximately 20% higher than
in a CR.
Selective hydrogen recovery from the reaction zone shifts the
position of equilibrium to the products side and improves conver-
sion of methanol. According to the Reaction (1) it is released 1.5
times more hydrogen on one mole of converted methanol than
according to the Reaction (2). This increases hydrogen yield and
rise of CO2 to CO ratio in the reaction products. For example in the
adsorption of permeable species (in this case H ) due to the pres-
2
ence of some less permeable or non-permeable substances, thus
reducing the overall efficiency of the process. In the case of thin foils
this effect, called a concentration polarization can be quite substan-
tial. This, unfortunately, decreases the efficiency of the membrane
catalysis. It is worth to highlight, that hydrogen, obtained in the
permeate side was ultrapure in all cases.
case of DND-based catalyst the amount of released CO was close to
◦
5
% at ∼330 C in the conventional reformer. At the same time in the
MR this value decreased to 1.5%, indicating the selectivity promo-
tion. Decrease of CO quantity in the MR takes place in the case of
all catalysts (Fig. 12a–c).
The maximum hydrogen recovery degree for the membrane
with a thickness of 70 microns on the Ru–Rh/DND catalyst reached
4. Conclusions
1
5% only. It could be expected that the process efficiency will
Ni–Cu/DND, Ru–Rh/DND, Ru–Rh/PAN, Ru–Rh/Vulcan catalysts
increase significantly when a membrane with thickness of 12
microns will be used. However, hydrogen permeation through
this membrane was not reversely proportional to the membrane
thickness and increased no more than twice. When the membrane
having a thickness of 12 microns was used the maximum degree
of hydrogen recovery reached only 25%. Moreover this value was
slightly varied in the investigated temperature range.
The mechanism of hydrogen permeation through the Pd-
containing membrane includes dissociative adsorption of its
molecules and recombination desorption of hydrogen atoms on
the entrance and exit surfaces of the membrane, respectively, and
diffusion through the membrane body. For relatively thick foil dif-
fusion in the membrane bulk is the rate-limiting step. Therefore, the
flow of hydrogen is reversely proportional to the thickness of the
membrane and proportional to the difference between the square
for the process of methanol steam reforming were synthesized.
Achieved methanol conversion was not lower than 85% for all cata-
◦
lysts at temperatures of about 330 C. Catalyst based on Ru–Rh alloy
differs with significantly higher activity as compared with Ni–Cu.
At the same time, at low temperatures Ni–Cu/DND catalyst is more
selective.
Although a pure carrier is inactive in MSR, its nature has a sig-
nificant effect on the activity of the catalytic system. Activity of the
catalysts decreased in the line of DND > PAN > Vulcan. At the same
time, carbon black Vulcan supported catalyst was the most selective
toward to hydrogen production. Maximum amount of hydrogen at
◦
3
2
00–310 C was obtained on the DND supported catalyst, which is
.3 moles from 3 theoretically possible. Surface of nanodiamonds is
rich with oxygen-containing groups, namely OH,
C O, COOH,
COOR. Perhaps the presence of these species determines a higher
Please cite this article in press as: A.A. Lytkina, et al., Bimetallic carbon nanocatalysts for methanol steam reforming in conventional
and membrane reactors, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.01.003

G Model
CATTOD-9958; No. of Pages8
ARTICLE IN PRESS
A.A. Lytkina et al. / Catalysis Today xxx (2016) xxx–xxx
8
activity of the supported catalyst DND/Ru–Rh. Their concentration
is negligible in the case of PAN and carbon black Vulcan.
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the processes proceeding above the membrane surface such as
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Please cite this article in press as: A.A. Lytkina, et al., Bimetallic carbon nanocatalysts for methanol steam reforming in conventional
and membrane reactors, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.01.003